Abstract

Regional gravity and magnetic data from the Irish Midlands have been studied in an attempt to map the structures within and beneath the Lower Carboniferous Limestones. The magnetic data suggest that the Precambrian crystalline basement is at a depth of 3–4 km and is overlain by Ordovician volcanics of varying magnetization. A Fourier analysis of the gravity field has been used to construct a residual gravity map which represents principally the gravity effect of shallow sources. These sources are probably the topographic variations of dense Ordovician volcanics beneath a less dense succession of Ordovician to Carboniferous sediments.

The residual gravity clearly shows the presence of a series of linear volcanic blocks and sedimentary troughs trending NE–SW. The major mineralized zones in the area all occur close to the margins of these troughs. This investigation indicates that the initial position of the Midland Plain was controlled by Caledonide factors, some of which modified the later subsidence which we attribute to Hercynian plate-margin processes. The genesis of the major ore-bodies is probably associated with de-watering of the shale-filled troughs, the subsequent concentration of metal-bearing fluids along the trough margins and their deposition in relatively unloaded carbonate host rocks on the up-standing blocks.

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